You specify the transaction commit mode in the XML deployment descriptor – specifically the vendor specific XML (file ejb-inprise.xml). It is specified as a property on an entity bean and goes like this

  <property>
            <prop-name>ejb.transactionCommitMode</prop-name>
            <prop-type>Enumerated</prop-type>
            <prop-value>A</prop-value>
   </property>